1. Field of the Invention
The present invention generally relates to phase change memory, typically referred to as PRAM. More specifically, a heater element external to a phase change material (PCM) element, in combination with using only a thin layer of the PCM element for storing the information state of the memory unit, greatly improves speed and controllability.
2. Description of the Related Art
Up until the advent of the present invention described herein, phase change memory (phase change random access memory or PRAM) has been considered as a relatively slow, but nonvolatile, form of memory in a similar category to FLASH memory. It would be highly desirable to design a PRAM which has a much higher performance and is capable of competing as a nonvolatile memory in the role of DRAM, including embedded applications.
Dynamic random access memory (DRAM), the most widely used form of memory in computers, has several well-known disadvantages, including the characteristic that memory is lost when the computer or device is switched off. DRAM is also relatively slow. Moreover, it would be desirable to significantly reduce the memory footprint (e.g., the area occupied by the memory cell).
Phase change memory, or PRAM, is based on a phase change material (PCM), which may be in one of two states, which, in the case of chalcogenide glass PCMs, are the amorphous and crystalline phase. The same phases are exploited in optical CD-RW and DVD-RW disk technologies. Commonly-used chalcogenide materials are germanium antimony tellerium (GeSbTe) alloys, usually referred to as “GST”, and a specific current commonly-used GST is Ge2Sb2Te5, referred to herein as “GST 225”.
In one embodiment of this invention, a form of GST is used as the PCM. As used herein, the term “GST” refers generically to a PCM containing at least one of the elements Ge, Sb or Te, and may include compositions of the form GenSbmTep, where n,m and p are integers or fractions defining the GST composition, or any binary combinations (e.g., GenSbm, SbmTep, GenTep), or some GST compositions doped with additional elements (e.g., N). Although GST is used as an illustrative embodiment of the present invention, other PCM can also be used for practicing this invention. Thus, the PCM may comprise other materials, including a completely novel material, which may or may not contain any of the elements Ge, Te, or Sb.
Switching between the states is done by thermal cycling, as shown in the “set” and “reset” processes 100 shown in FIG. 1. Amorphous-to-crystalline conversion involves an anneal below melting point (“Set” process 101), while crystalline-to-amorphous conversion involves melting followed by a fast quench (“Reset” process 102). The READ process exploits the much higher electrical resistivity of the amorphous state. That is, the information state of the memory cell is read by sensing the amount of current flow due to application of a standard voltage.
Current designs of PRAM achieve thermal cycling by joule heating caused by passing current through the PCM. However, for the DRAM application, this method has several disadvantages, including:
1. When the PCM is in the amorphous, high-resistance, state, passing significant current depends on an irreversible electrical breakdown-like conduction phenomenon. The control of this switching process is poor because of the non-linearity of this mechanism.
2. The geometry of the joule-heating based PRAM cell is not optimal for a high performance PRAM.
3. The irreversible conduction approach is only supported by a narrow range of GST compositions.
4. There is no simple scaling law.
Thus, because of these known disadvantages, a need exists for an improved switching memory cell based on phase change (melting/recrystallization) to address at least some of these aspects. Preferably, an improved memory cell would have characteristics, including speed, to be competitive with and even superior to current DRAM.